The present invention relates to a catalyst for polymerizing .alpha.-olefins, more specifically a catalyst composed of a titanium composition formed by heating a co-comminuted mixture of a magnesium halide and a complex of a aluminum halide and a carboxylic acid ester with titanium tetra-chloride; and an organic aluminum compound, and optionally added thereto at least one component selected from an alkyl aluminum halide, a carboxylic acid ester and its complex formed with an aluminum halide.
Previously, in, for example, propylene polymerization various kinds of improved catalysts containing commercially available titanium trichloride and diethyl aluminum monochloride as the major components thereof have been used; in such cases, the weight ratio of a residual polypropylene to the totally resultant polymer, determined by a boiling n-heptane extraction test, (hereinafter referred to as "Total II") was e.g. 90.about.95 wt. %, but the polypropylene as obtained by the polymerization was liable to contain 30.about.200 ppm of titanium.
Properties of polypropylene such as weather-proofness, heat resistance and color shade vary depending on the nature of stabilizer which may be used and these properties become worse if a transition metal such as titanium has been left therein.
In order to avoid this problem, the resultant polymer has been commercially sold after subjecting the catalyst used with, for example, an alcohol to decomposition followed by aftertreatments such as washing and deashing. In the case where high quality polypropylene is required, the aftertreatment has to be carried out so that the titanium content of same is lowered to about 3.about.10 ppm in general.
Therefore, a catalyst having such an improved activity that yield of polypropylene may be increased up to about 100.about.300 Kg/g-Ti (corresponding to 3.about.10 ppm of titanium content contained in the polymer) and capable of giving a polypropylene having higher total II, is required in order to effect the so-called non-deashing process which does not need the aftertreatment mentioned above.
Recently, a process for greatly improving polymerization activity of a Ziegler catalyst by supporting the titanium component of the Ziegler catalyst on a carrier has been developed and generally used as a polymerization catalyst for ethylene. However, in the case where a well-known carrier type catalyst is used for the polymerization of .alpha.-olefins such as propylene, butene and a higher homologue, it is difficult to obtain a polymer having an isotactic structure and the use of the catalyst is not practical because useful polymers having higher crystallinity cannot be obtained.
The carrier type catalyst having higher polymerization activity only as used in ethylene polymerization is not suitable for the purpose of realizing the non-deashing process in an .alpha.-olefin polymerization; and it is a technically important problem to develop a catalyst capable of maintaining the crystallinity of the resultant polymer as well as having high catalytic activity.
Processes are disclosed in Japanese Laid-Open Patent Publication Nos. 9342/72, 16986/73, 16987/73, 16988/73 and 86482/74, for improving the crystallinity of a resultant polymer by adding an organic compound of an electron donor type as the third component to a carrier type catalyst comprising a titanium compound supported on a magnesium halide and tri-alkyl-aluminum.
In the polymerization of propylene with a catalyst of a two-component system comprising titanium component supported on a carrier and tri-alkyl-aluminum compound, the crystallinity of the resultant polymer is extremely low despite the high polymerization activity of the catalyst.
Although the crystallinity of the resultant polymer is improved by adding an organic compound of an electron donor type to the catalyst, the aforementioned process is not practically satisfactory due to the fact that the polymerization activity of the catalyst is considerably lowered.
Another problem in the aforesaid three component system catalyst resides in the fact that the activity of the polymerization catalyst becomes higher with a large amount of tri-alkyl-aluminum, but the crystallinity of the resultant polymer becomes lower; and conversely the activity of the catalyst becomes lower when a smaller amount of tri-alkyl-aluminum is used to improve, the crystallinity of the resultant polymer. Further, when an alkyl aluminum halide of the formula: EQU AlRmX.sub.3 --m
(wherein R stands for an alkyl radical and m is a number defined by 1.5.ltoreq.m.ltoreq.3) is used as an organic aluminum compound in lieu of tri-alkyl-aluminum, both the polymerization activity of the catalyst and the crystallinity of the resultant polymer are low and the catalyst cannot be practically used.
There is disclosed in Japanese Laid-Open Patent Publication No. 9342/72 the use of a hydride of a metal in the Groups I.about.III of the Periodic Table or an organic compound containing a metal as a second component to be combined with titanium component supported on a carrier. However, when the alkyl aluminum halide was used, both the polymerization activity of the catalyst and the crystallinity of the resultant polymer were very low as will be referred to hereinafter, and the catalyst could not be practically used.
In the Japanese Laid-Open Patent Publication Nos. 16986/73, 16987/73, 16988/73 and 86482/74, there is disclosed only the use of trialkyl aluminum as the second component.